1. Field of the Invention
The present invention relates generally to a stator assembly for a turbo-machine, and more specifically to a damped stator assembly.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
An axial flow compressor in a gas turbine engine includes a row of stator vanes upstream and downstream of a row of rotor blades, the stator vanes functioning to guide the air flow into the rotor blades for increasing the efficiency and diffusing the air to increase static pressure. The stator vanes include an arrangement of airfoils that extend between an outer shroud and an inner shroud that defines the flow path through the stator assembly. The stator assembly is also subject to vibrations and thermal stress from high temperature exposure. For these reasons, prior art stator assemblies are formed as segments with one, two, three or more vanes in each segment. Adjacent stator segments are separated by a relief slit to allow for thermal expansion to allow free vibration of the shrouded segments against frictional damper springs, and thus reducing vibratory stresses and extending the life of the part. However, these relief slits allow for leakage of the fluid through the stator assembly. When the segment includes a single vane, then the leakage flow area is larger.
To provide damping to a segmented stator assembly, damper springs are used on the inner shroud area to provide damping. The stator segments are attached to the engine casing at the outer shroud location while the inner shroud is unsupported in the turbo-machine. The stator vanes can vibrate in several different modes. The relief slits separate adjacent segments and prevents the vibratory modes from causing destructive amplification in adjacent vane segments. This amplification can lead to cracking and catastrophic failure of the gas turbine engine if the crack is not detected in time. The prior art U.S. Pat. No. 7,291,946 B2 issued to Clouse et al. on Nov. 6, 2007 and entitled DAMPER FOR STATOR ASSEMBLY discloses a stator assembly with a damper spring positioned between an inner surface of the inner shroud and an outer surface of a seal. The damper spring rubs against the seal surface to dampen the stator assembly. In the Clouse et al. invention, the stator assembly is formed from many segments with many relief slits therein. A relief slit exists for every vane in this design. Therefore, the leakage flow is relatively high.
Another disadvantage of relief slits is caused by the mass of the shroud supported by an individual vane. This mass tends to reduce the resonant frequency modes of a vane compared to a similar vane with no attached shroud. Higher resonant frequencies are desirable, possibly avoidable potential drivers under operational conditions.
Another disadvantage yet is the high cost in fabricating the relief slits. Because the leakage is undesirable, the relief slits are held to a close tolerance fabrication to minimize the leakage area. To form close tolerance relief slits, wire electro-discharge machining (EDM) is used to cut the slits which is both time consuming and expensive. Also, removal of the re-melted material subsequent to the cutting further increases the cost of fabricating the relief cuts. This operation is difficult and is often done manually. Further increasing the cost still yet is the cost involved with inspecting the features which is difficult since the relief slits tend to be very narrow and often at compound angles relative to the full ring.